Process for the storage of hydrogen using a system that strikes a balance between an alloy of alkaline metal and silicon and the corresponding hydride

ABSTRACT

A process for the reversible storage of hydrogen, comprising bringing an alloy of alkaline metal and silicon into contact with gaseous hydrogen leading to the formation of the hydride or corresponding hydrides, comprises the use of at least one balanced system that corresponds
         to the formula:
 
M X     M   Si  M X     M   SiH n  
    where M is selected from among Li, Na, or K and in which atomic ratios X M  take on the following values:
 
X Li =1
 
1≦X Na ≦3
 
1≦X K ≦2
    n is the number of hydrogen atoms corresponding to the stoichiometry of the hydride or formed hydrides.   or to the formula
 
MSi X     Si      MSi X     Si   H 2X     Si     +1  
    where M is selected from among Li, Na, or K and in which the atomic ratio X Si =Si/M takes on a value of 1 to 4.

This application is a continuation of U.S. patent application Ser. No.11/347,576, filed, Feb. 6, 2006, now abandoned which claims the priorityof French Application No. 05/01.231, filed on Feb. 7, 2005.

FIELD OF THE INVENTION

This invention relates to a process for reversible storage of hydrogenusing new materials that are potentially advantageous for the storage ofhydrogen.

PRIOR ART

Within the context of research of new energy systems, the development ofprocesses for storage and transport of hydrogen is very important.Compounds with a lithium base that are used for storing hydrogen areknown. Due to an excessive stability of the lithium hydride that makesit difficult to store hydrogen, it is necessary to use more complexlithium hydrides.

Document U.S. Pat. No. 6,514,478 B2 describes Li—Be—H-type hydrides. Ina very general way, Patent Application WO 2004/05070 A2 describes theuse of anisotropic nanostructures such as, for example, lithium nitride,able to be used in devices for storing hydrogen.

Object of the Invention

This invention relates to a process for reversible storage of hydrogenusing new materials that are potentially advantageous for the storage ofhydrogen (theoretically more than 5% by mass) under the followingconditions, defined by the pressure-temperature isothermal plateau:

-   -   270 K<T<370 K    -   and 1<P<10 atm (or about 0.1 MPa<P<about 10 MPa).

These new materials comprise a balanced system that is formed between analloy of alkaline metal and silicon and hydride or the correspondinghydrides; they are of the type:M_(X) _(M) Si

 M_(X) _(M) SiH_(n)where M is selected from among Li, Na, or K and in which the atomicratios X_(M) take on the following values:X_(Li)=11≦X_(Na)≦31≦X_(K)≦2

n is the number of hydrogen atoms corresponding to the stoichiometry ofthe hydride or formed hydrides.

If the alloy that is formed between the alkaline metal and silicon issuperstoichiometric, i.e., if the ratio X_(Si) that is defined by Si/Mtakes on values of from 1 to 4, the balanced system that is used is asfollows:MSi_(X) _(Si)

 MSi_(X) _(Si) H_(2X) _(Si) ₊₁

The new materials that are used from the hydrogen storage process aremore particularly of the type:Li_(i)Si

 Li_(X) _(L) SiH₃,Na_(X) _(Na) Si

 Na_(X) _(Na) SiH_(n) andK_(X) _(K) Si

 K_(X) _(K) SiH_(n).

n is the number of stoichiometric hydrogen atoms of the hydride orhydrides that are formed.

Atomic ratios X_(M) take on the following values:X_(Li)=11≦X_(Na)≦31≦X_(K)≦2.

The invention also relates to new structures that correspond to formulasNaSiH₃ and LiSiH₃.

DESCRIPTION OF THE DRAWINGS

FIG. 1 plots the values of ΔE_(hyd) that are calculated and theexperimental values ΔH_(hyd) of the literature.

FIG. 2 is the Van't Hoff diagram of simple hydrides using the calculatedvalues of ΔE_(hyd).

FIG. 3 is the X-ray diffraction spectrum of the NaSiH₃ crystallinestructure.

FIG. 4 is the X-ray diffraction spectrum of the LiSiH₃ crystallinestructure.

FIG. 5 is the Van't Hoff diagram for alloys of lithium and silicon.

FIG. 6 is the Van't Hoff diagram for the alloys of sodium and silicon.

FIG. 7 is the Van't Hoff diagram for the alloys of potassium andsilicon.

DETAILED DESCRIPTION OF THE INVENTION

In the storage process according to the invention, the alloys ofalkaline metal and silicon are brought into contact with gaseoushydrogen and thus lead to the formation of (a) corresponding hydridecompound(s) (hydrogen absorption). By slightly increasing thetemperature or slightly reducing the hydrogen pressure, the formedhydride restores the hydrogen (desorption). It is therefore a reversiblestorage process.

The hydrides at equilibrium with the alloys KSi, NaSi or LiSi or withthe elements K, Li or Na and Si have improved thermodynamic propertiesfor the storage of hydrogen.

The KSiH₃ structure is known, whereas the NaSiH₃ and LiSiH₃ structuresare not inventoried in the ICSD and CRYSMET crystallographic bases, towhich we have access. The NaSiH₃ and LiSiH₃ structures will therefore beresolved analogously to the KSiH₃ structure by the method of calculationthat is described below.

-   -   The ICSD (Inorganic Crystal Structure Database) base is the        property of the “Fachinformationszentrum Karlsruhe [Technical        Information Center of Karlsruhe] (FIZ)” in Germany and the        “National Institute of Standards and Technology (NIST)” in the        U.S.A.    -   The CRYSMET base belongs to and is maintained by “Toth        Information Systems,”Ottawa, and le Conseil national de        recherches [National Research Council] of Canada.        (ICSD and CRYSMET can be accessed within the MedeA interface        marketed by Materials Design S.a.r.l., Le Mans (France)).

Many useful properties of a solid material can be derived directly fromdetermining its chemical cohesion energy. This cohesion energy isintrinsically based on the chemical composition, the local atomicstructure of the material, its electronic properties and all thephysical properties that are derived therefrom. Quantum physics and morespecifically the density functional theory (whose abbreviation DFT isobtained from the English “Density Functional Theory”) provide areliable base for the quantitative prediction of structural, electronicand thermodynamic properties of an atomic, molecular or crystallinestructure before any attempt at synthesis of the laboratory material(see: W. Kohn, L. J. Sham, Phys. Rev. A 140, 1133 (1965)). Inparticular, the formalism of the DFT, as it is implemented in manycurrent quantum software applications, such as:

-   -   the “Vienna Ab initio Simulation Package” (VASP) (see: G.        Kresse, J. Hafner, Phys. Rev. B 48 (1993) 13115; G. Kresse, J.        Furthmüller, Phys. Rev. B 6 (1996) 15; as well as the address        URL: http://www.cms.mpi.univie.ac.at/vasp/; references [1]);    -   “CASTEP” (see: http://www.tcm.phy.cam.ac.uk/castep/), and    -   “Gaussian” (see: http://www.gaussian.com),        has as a central object the determination of the electronic wave        function of a material that is simulated by an approximate        solution to the famous Schrödinger equation. Access to the wave        function makes possible the development of a predictive and        quantitative methodology of the chemical bond in an atomic,        molecular or crystalline structure.

In the search for new materials for the storage of hydrogen, theexperimenters need to rely on the knowledge and a methodology of thechemistry of the solid. On the basis of thermodynamic concepts such asthe formation enthalpy, the relative stabilities of the structures ofmaterials can be quantified based on temperature and pressureconditions. The modern techniques of quantum calculation such as the DFToffer the advantage of relying on a minimal knowledge of empirical datafor determining these same thermodynamic properties. Thanks to theknowledge of basic constants of physics, these techniques, thus oftencalled “ab initio,” therefore make it possible to predict the energystability and the physico-chemical properties of a crystalline structuredefined by its composition and its crystallographic mesh, independentlyof any experimental approach. Moreover, these techniques make itpossible to eliminate experimental uncertainties on the structure of amaterial.

The use of intermetallic hydrides as materials for storing hydrogen isbased on the following chemical equilibrium:

$\begin{matrix}\left. {{\frac{2}{n}M} + H_{2}}\rightarrow{\frac{2}{n}{MH}_{n}} \right. & (1)\end{matrix}$where M represents the stable metallic phase being transformed into thestoichiometric hydride phase MH_(n).

This hydride phase has a theoretical mass storage capacity that is equalto nMH/(nMH+MM)×100%, where MH is the molar mass of atomic hydrogen andMM is that of metal.

The thermodynamic characteristics of transformation (1) are described bya pressure-temperature isotherm. When the two hydride and metal phasesco-exist, the isotherm has a plateau. Temperature T and equilibriumpressure P_(eq) of the plateau are determined by the Van't Hoffequation:

$\begin{matrix}{{\frac{2}{n}{\ln\left( \frac{P_{eq}}{P^{0}} \right)}} = {\frac{\Delta\; H_{hyd}}{RT} - \frac{\Delta\; S_{hyd}}{R}}} & (2)\end{matrix}$where:

ΔH_(hyd) (or ΔS_(hyd)) represents the enthalpy variation (or the entropyvariation) of transformation (1);

R=8.314510 J.mol⁻¹.K⁻¹ is the molar constant of the ideal gases, and

P⁰=1 bar is the standard pressure (or 0.1 MPa).

This approach can be generalized for hydrides of metal alloys, AB_(x),in the following way:

$\begin{matrix}\left. {{\frac{2}{n}{AB}_{x}} + H_{2}}\rightarrow{\frac{2}{n}{AB}_{x}H_{n}} \right. & (3)\end{matrix}$where A and B are two metal elements and x is the atomic ratio B/A inthe alloy.

It is commonly recognized that the primary contribution according to theterms of entropic variation ΔS_(hyd) is the loss of entropy of thehydrogen molecule that passes from the gas phase in an adsorbed stateinto the solid state of the final hydride. The value of ΔS_(hyd) isknown for being close to 130 J. K⁻¹.mol⁻¹ of H₂, regardless of thehydride (see: “Hydrogen-Storage Materials for Mobile Applications,” L.Schlapbach, A. Züttel, Nature 414 (2001) 353-358, reference [5]; and“Hydrogen Storage Properties of Mg Ultrafine Particles Prepared byHydrogen Plasma-Metal Reaction,” H. Shao, Y. Wang, H. Xu, X. Li,Materials Science Engineering B 110 (2004) 221-226, reference [6]).Below, we retained this value. According to equation (2), also valid forreaction (3), the logarithm of the pressure at equilibrium, P_(eq),varies linearly with the inverse of temperature T. The slope of thelinear relationship is determined by ΔH_(hyd). In the followingexamples, we will show, thanks to the Van't Hoff diagrams, thevariations of the logarithm of P_(eq) based on 1/T (more specifically1000/T for reasons of providing units). Such diagrams make it possibleto identify potentially advantageous materials for storing hydrogen in atargeted range of P_(eq) and T.

Consequently, the prediction (by a reliable theoretical approach) is ofmajor interest for the knowledge of temperature and pressure conditionsin which the metal or alloy can be transformed into hydride. As ΔH_(hyd)is in general exothermic (for the stable hydrides), the slope isnegative. The value of ΔH_(hyd) closely depends on the stability of thehydride relative to the metallic phase or to the alloy: the morethermodynamically stable the hydride, the more reaction (1) or (3) isexothermic.

The formation enthalpy of the hydride, ΔH_(hyd), can be expressed basedon the internal energy variation during hydrogenation, ΔE_(hyd):ΔE _(hyd) =E _(AB) _(x) _(H) _(n) −E _(AB) _(x) −E _(H) ₂   (4)where E represents the internal energy of the hydride phases, metal, andthe hydrogen molecule in gaseous phase. The internal energy of thematerial is linked to interactions between the atomic centers thatconstitute the material and the electrons. This energy is also oftencalled electronic energy and is directly connected to the cohesionenergy of the material. The expression of ΔH_(hyd) based on ΔE_(hyd) isas follows:ΔH _(hyd) =ΔE _(hyd) +PΔV+ΔZPE+TΔc _(p)  (5)where

Δc_(p) represents the calorific capacity variation between the hydridephase and the metal phase,

ΔZPE is the energy variation at the zero point between the hydride phaseand the metal phase,

and ΔV is the variation of molar volume between the hydride phase andthe metal phase.

The modern techniques for quantum simulation make it possible tocalculate systematically the values of E_(AB) _(x) _(H) _(n) , E_(AB)_(x) , and E_(H) ₂ , and therefore to derive therefrom the value ofΔE_(hyd). For a given crystalline phase (known or unknown in anexperimental way), the initial crystallographic structure is determinedby the space group, the parameters of the primitive cell, and the atomicpositions in the mesh of the primitive cell. For existing structures,the bases of crystallographic data, such as ICSD and CRYSMET, providethis information.

For the new structures (unknown or not totally resolved experimentally),the same standard description will be adopted in this invention. We willalso add the simulation of the X-ray diffraction spectrum (DRX),commonly used experimentally for characterizing the observed structures.

For any structure (known or new), the process of rigorous simulation isadopted so as to determine the so-called basic state of the structure,i.e., the stable state of the structure. In this basic state, the valuesof E_(AB) _(x) _(H) _(n) , E_(AB) _(x) , E_(H) ₂ , and ΔE_(hyd) arecalculated. This process makes it possible in particular to determinethe electronic wave function of the system by optimizing the crystallinestructure for the hydride and metal solids and the hydrogen molecule,thanks to modern quantum simulation techniques at the DFT level,accessible in software such as VASP (see references [1] above). For thispurpose, the following criteria are imposed during the calculation:

-   -   the criterion of convergence of the electronic energy should be        set at 0.01 kJ/mol of primitive cell,    -   the criterion of convergence of the atomic positions and the        volume of the primitive cell of the solid should lead to an        energy precision of 0.1 kJ per mol of primitive cell,    -   the grid of points-k used to describe the Brillouin zone should        be large enough to ensure a fluctuation of the electronic energy        that is weaker than 0.01 kJ per mol of cell,    -   the size of the plane-wave base that is used or the precision of        the base that is used should ensure a convergence of the        electronic energy of more than 0.1 kJ per mol of primitive cell.

For the applications of storage of on-board hydrogen, an equilibriumtemperature of close to 300 K (1000/T # 3.3 K⁻¹) is generally sought fora pressure that is close to 1 atm (about 0.1 MPa). Due to equation (2),this corresponds to a value of ΔH_(hyd) that is close to −39 kJ per molof hydrogen. For this invention, and because of the precision of thesimulation approach defined above, we will designate materials that arepotentially advantageous for storing hydrogen, all the materials whoseisothermal plateau verifies the following conditions:270<T<370 K (or 2.7<100/T<3.7 K⁻¹)and1<P_(eq)<10 atm (or about 0.1 MPa<P_(eq)<about 10 MPa)  (6).

The target window that embodies this domain will be shown in all theVan't Hoff diagrams in the following examples.

According to the invention, the alkaline metal that is selected can beof “mixed” type, in which lithium, sodium and potassium can besubstituted respectively by sodium and/or potassium, lithium and/orpotassium and lithium and/or sodium.

According to the invention, the alloy can also comprise, in a proportionof less than 5% by weight, at least one light transition metal of groups3 to 12 of the periodic table selected from among, for example, Sc, Ti,V, Cr, Mn, Fe, Co, Ni, Cu and Zn.

The alloy of alkaline metal and silicon of the invention can come insolid or in dispersed form, obtained by, for example, grinding.

The process is applied to, for example, the storage of on-board,stationary or portable hydrogen.

The invention also relates to new structures that correspond to formulasNaSiH₃ and LiSiH₃, whose X-diffraction spectra are provided respectivelyin FIGS. 3 and 4.

EXAMPLES

Among the following examples, Example 1 is provided by way ofcomparison, and Example 2 illustrates the invention.

Example 1 (For Comparison) Known Cases of Simple Hydrides

The diagram of FIG. 1 plots the values of ΔE_(hyd) that are calculatedaccording to the process that is described above and the experimentalvalues ΔH_(hyd) of the literature (see: “CRC Handbook of Chemistry andPhysics,” 76^(th) Edition 1995-1996, David R. Lide Editor-in-Chief, CRCPress).

The crystallographic structures that are used are those of hydride andmetal phases that are stable under conditions that are close to thoseset forth above in (6). They are recorded in Table 1.

TABLE 1 Simulated Structural Properties and Mass Capacity of SimpleHydrides. Crystallographic Space % by Balance Hydride Reference GroupMass Equation LiH ICSD.61751 FM3-M 22.37 2Li + H₂ → 2LiH NaH ICSD.33670FM3-M 8.00 2Na + H₂ → 2NaH BeH₂ ICSD.84231 IBAM 18.17 Be + H₂ → BeH₂MgH₂ ICSD.26624 P42/MNM 7.60 Mg + H₂ → MgH₂ CaH₂ ICSD.23870 PNMA 4.75Ca + H₂ → CaH₂ YH₂ CRYSMET.36093 Fm-3m 2.20 Y + H₂ → YH₂ TiH₂CRYSMET.38081 Fm-3m 4.01 Ti + H₂ → TiH₂ ZrH₂ CRYSMET.39242 I4/mmm 2.15Zr + H₂ → ZrH₂

The result of FIG. 1 shows that there is a linear relationship betweenthe two basic values—experimental Δ_(hyd) and calculated ΔE_(hyd)—on abroad range of representative hydrides. These examples also show thatthe calculated value ΔE_(hyd) is a good thermodynamic descriptor forpredicting thermodynamic properties of materials for the purpose ofstoring hydrogen. The final precision on the energy is on the order of 3to 5%, which is in agreement with the method of calculation used and theprocess described above. This result implies that the contributions thatare linked to terms Δc_(p), ΔZPE and ΔV are negligible compared to thecontributions of cohesion energies.

FIG. 2 represents the translation of these values to the Van't Hoffdiagram by using the calculated values of ΔE_(hyd). As is knownexperimentally, none of the simple hydrides of Table 1 (except for BeH₂,which exhibits other difficulties of operation) makes it possible tocome close to the target window that is defined above, which makes itpossible to consider the use of these materials for storing hydrogen.

For example, the case of magnesium hydride, which is used as a referenceto the following, reveals that ΔH_(hyd)(MgH₂) is equal to −75.0 kJ permol of H₂ (see references [2] and [3] above). The calculation provides avery close value, on the order of −70.2 kJ per mol of H₂. Theequilibrium temperature at atmospheric pressure is 575 K experimentally(see reference [2]), which is much too high to be usable.

Example 2 Cases of Silicon Hydrides

A new family of materials that are based on silicon is potentiallyadvantageous for storing hydrogen: KSiH₃, LiSiH₃ and NaSiH₃. Theydevelop a high mass storage capacity (see Table 2). Only the KSiH₃ phaseis inventoried in the database ICSD No. 65954 with space group PNMA. Forthe NaSiH₃ and LiSiH₃ phases, the structure of prototype KSiH₃ isretained (see Table 2).

TABLE 2 Structures of Silicon-Potassium, Silicon-Lithium andSilicon-Sodium Hydrides Crystallographic Formula Reference Space Group %by Mass KSiH₃ CRYSMET.65954 PNMA 4.27 Prototype KSiH₃ LiSiH₃CRYSMET.65954 PNMA 7.89 Prototype KSiH₃ NaSiH₃ CRYSMET.65954 PNMA 5.55

The X-diffraction spectra of the two new structures NaSiH₃ and LiSiH₃are provided in FIGS. 3 and 4 respectively.

TABLE 3 Definition of the NaSiH₃ Structure Space Group PNMA Parametersof the a = 8.29396 b = 4.93395 c = 6.08397 Monoclinical Cell α = 90.00 β= 90.00 γ = 90.00 Element X Y Z Na (4c) 0.16982 0.25000 0.16436 Si (4c)0.05798 0.25000 −0.34777 H (4c) −0.11812 0.25000 −0.26924 H (8d) 0.108910.47835 −0.18539

TABLE 4 Description of the Simulated LiSiH₃ Structure Space Group PNMAParameters of the a = 8.53326 b = 4.71238 c = 5.30609 Monoclinical Cellα = 90.00 β = 90.00 γ = 90.00 Element X Y Z Li (4c) 0.17456 0.250000.15957 Si (4c) 0.08540 0.25000 −0.35280 H (4c) −0.06323 0.25000−0.19195 H (8d) 0.16563 0.48443 −0.19051

The chemical balances considered and the calculated values of ΔE_(hyd)for the different materials are as follows:2/3K+2/3Si+H₂→2/3KSiH₃ ΔE _(hyd)=−55.3 kJ/mol  (a)2/3KSi+H₂→2/3KSiH₃ ΔE _(hyd)=−45.5 kJ/mol (x_(K)=1)2/3Li+2/3Si+H₂→2/3LiSiH₃ ΔE _(hyd)=−34.8 kJ/mol  (b)2/3LiSi+H₂→2/3LiSiH₃ ΔE _(hyd)=−7.3 kJ/mol (x_(Li)=1)2/3Na+2/3Si+H₂→2/3NaSiH₃ ΔE _(hyd)=−36.2 kJ/mol  (c)2/3NaSi+H₂→2/3NaSiH₃ ΔE _(hyd)=−31.9 kJ/mol (x_(Na)=1)

These balances can be expressed in the following general way by assuminga variable composition of M/Si=x_(M) (with x_(M)≧1):2/(2+x _(M))MSi+2(x _(M)−1)/(2+x _(M))M+H₂→2/(2+x _(M))MSiH₃+2(x _(M))MH

In the case of a superstoichiometric material of Si (x_(Si)=Si/Mencompassed between 1 and 4), the balance can be expressed in thefollowing manner:2/(2x _(si)+1)MSi+2(x _(Si)−1)/(2x _(Si)+1)Si+H₂→2/(2x _(Si)+1)MSi_(X)_(Si) H_(2X) _(Si) ₊₁

This equilibrium involves the formation of a single hydride compound ofthe di-, tri- or tetra-silyl type of alkaline metals (M=Li, Na, K).

The KSi, LiSi and NaSi alloy phases are the structures that areidentified in the CRYSMET base and provided in Table 5.

TABLE 5 Structures of Silicon Alloys Considered Crystallographic FormulaReference Space Group KSi CRYSMET.84809 P-43n LiSi CRYSMET.102320 I41/aNaSi CRYSMET.80184 C2/c

The Van't Hoff diagrams that correspond to various preceding equilibriaand for various elements are provided in FIGS. 5, 6 and 7 for theelements Li, Na and K, respectively. These diagrams show that thehydrides that contain silicon in the presence of lithium, sodium orpotassium are likely to be advantageous materials with a high masscapacity for storing hydrogen under favorable thermodynamic conditions.

Table 6 below indicates the mass contents and hydrogenation enthalpybased on x_(M) as defined by the preceding chemical balances:

TABLE 6 X_(M) % by Mass ΔE_(hyd) (kJ/mol) Li_(X) _(Li) Si 1 7.89 −7.3 28.70 −49.3 3 9.27 −74.4 4 9.70 −91.2 Na_(X) _(Na) Si 1 5.55 −31.9 2 5.12−47.3 3 4.90 −56.5 4 4.76 −62.7 K_(X) _(K) Si 1 4.27 −45.52 2 3.63−59.26 3 3.32 −67.51 4 3.15 −73.01

1. A process for reversible storage of hydrogen comprising bringing intocontact an alloy of a potassium and silicon, KSi having acrystallographic reference CRYSMET 84809 and a space group P-43n withgaseous hydrogen, under the conditions of 270K<T<370 K and 1<P<10 atm,leading to the formation of a crystalline hydride KsiH3 having acrystallographic reference CRYSMET 65954, under a balanced systemcorresponding to the formula:⅔KSi+H₂< - - - >⅔KSiH₃.
 2. A process for reversible storage of hydrogencomprising bringing into contact an alloy of potassium and silicon, KSihaving a crystallographic reference CRYSMET 84809 and a space groupP-43n with gaseous hydrogen, under the conditions of 270K<T<370K and1<P<10 atm, leading to the formation of a crystalline hydride KsiH3having a crystallographic reference CRYSMET 65954, under a balancedsystem corresponding to the formula:⅔KSi+H₂< - - - >⅔KSiH₃ wherein the potassium is partially substituted bysodium.
 3. A process according to claim 1, wherein the alloy alsocomprises, in a proportion that is less than 5% by weight, at least onelight transition metal of groups 3 to 12 of the periodic table selectedfrom among Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.
 4. A processaccording to claim 1, wherein the alloy of potassium metal and siliconis in solid form.
 5. A process according to claim 1, wherein the alloyof potassium metal and silicon is in a dispersed form.
 6. A processaccording to claim 1, wherein the alloy of potassium metal and siliconis obtained by grinding.
 7. A process according to claim 1, applied tothe storage of on-board hydrogen.
 8. A process according to claim 1,applied to stationary storage.
 9. A process according to claim 1,applied to portable storage.
 10. A process according to claim 2, whereinthe alloy also comprises, in a proportion that is less than 5% byweight, at least one light transition metal of groups 3 to 12 of theperiodic table selected from among Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu andZn.
 11. A process according to claim 2, applied to the storage ofon-board hydrogen.
 12. A process according to claim 2, applied tostationary storage.
 13. A process according to claim 2, applied toportable storage.